I notice in a lot of decisions like this, a lot of ideas are floated by a lot of us that don’t know the entire, detailed story. I’m one of the worst offenders. It’s why I go into “academic” mode. i.e. I speak of acedemic concepts rather than the specific case.
In this case, if Obama created an EO with an expiriation date, then when that date comes, it has expired.
That is why they call them “expiration dates”. If someone sues to get it extended in court, the only way the court can entertain the suit is if there is some constitutional violation regarding the expiration date. Of course, that would be absurd in this case.
The system established by DHS did not have an expiration date.
The deferred action to each applicant had an expiration date, so accepted applicants were required to renew "deferred action" (a sort of imunity/work permit arrangement) periodically.
The system is now split into two parts. The new applicant part is gone. The renewal part lives on, per court order.
Federal Courts have many widgets, not just the constitution. The widget used to reject the new DHS policy of not accepting renewal applications is the Administrative Procedures Act (APA).